Additive Manufactured Thermoplastic-Aluminum Nanocomposite Hybrid Rocket Fuel Grain and Method of Manufacturing Same

ABSTRACT

A hybrid rocket solid fuel grain having a cylindrical shape and defining a center port is additive manufactured from a compound of thermoplastic fuel and passivated nanocomposite aluminum additive. The fuel grain comprises a stack of fused layers, each formed as a plurality of fused abutting concentric circular beaded structures of different radii arrayed defining a center port. During operation, an oxidizer is introduced along the center port, with combustion occurring along the exposed port wall. Each circular beaded structure possesses geometry that increases the surface area available for combustion. As each layer ablates the next abutting layer, exhibiting a similar geometry is revealed, undergoes a gas phase change, and ablates. This process repeats and persists until oxidizer flow is terminated or the fuel grain material is exhausted. To safety achieve this construction, a fused deposition additive manufacturing apparatus, modified to shield the nanocomposite material from the atmosphere is used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 12/052,934 filed on Mar. 21, 2008 entitled SolidFuel Grain for a Hybrid Propulsion System of a Rocket and Method forManufacturing Same, now issued U.S. Pat. No. ______, which claimspriority to the provisional patent application No. 60/896,296 filed onMar. 27, 2007 of the same title. The entire disclosure of each one ofthese documents is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to rocket propulsion systems andspecifically to hybrid rocket engines. There are three basic types ofchemical rockets in use today: liquid rocket engines that use liquidpropellants, solid rocket motors that use solid propellants, and hybridrocket engines that use a combination of liquid and solid propellants.

In a conventionally designed hybrid rocket engine, the fuel is stored inthe solid state, while the oxidizer is stored in either the liquid orgaseous state. Traditionally in most hybrid rocket engine designs, thesolid fuel is cast-molded, extruded, or in some instances machined intoa cylindrically shaped structure referred to as a fuel grain. The fuelgrain is designed and formed to feature one or more internal passagesrunning through its length. These passages are referred to as ports. Thefuel grain port or ports dually serve as the hybrid rocket engine'scombustion chamber or chambers, and through a gas phase change andablation process, the fuel source.

The fuel grain is conventionally housed within a metal orfiber-reinforced polymer composite motor case designed to withstand thepressures and elevated temperatures created during the combustionprocess. The motor case may also feature an internal liner made from ahigh-temperature material to create a thermal barrier to prevent damageor burn-through during the rocket engine's operation.

The motor case, with fuel grain installed, is attached to a forward captypically machined or cast from high-temperature metal alloys. Theforward cap forms the pre-combustion chamber and houses the oxidizerinjectors and ignition system. The aft end of the motor case is attachedto an assembly which forms the post combustion chamber and allows secureattachment to the rocket nozzle. The assembled motor case with fuelgrain installed, forward cap, and aft assembly with attached nozzle isconventionally referred to as the motor or solid section of the hybridrocket engine.

In a conventionally designed hybrid rocket engine, liquid or gaseousoxidizer is stored separately in an integrally formed pressure vessel ortank forward of the motor section within the rocket powered vehicle.However, in some designs, liquid or gaseous oxidizer may be storedadjacent to the motor section or even remotely on the vehicle.Conventionally, the tank or pressure vessel stored liquid or gaseousoxidizer is urged through a specially designed plumbing system,typically including a flow control valve to feed oxidizer through one ormore oxidizer injectors housed within the motor section forward cap; andin turn, through the fuel grain port or ports.

The motive force needed to urge the liquid or gaseous oxidizer throughthe oxidizer injector or injectors into the fuel grain port or portswith sufficient flow rate to support combustion may be generated by anyone of several means such as enabling a liquid to gas phase change,causing an exothermic reaction using a catalyst, employing a mechanicalboost pump, pre-pressurizing the oxidizer tank with an externallysupplied inert gas, or using an on-vehicle high pressure tank filledwith an inert gas to boost oxidizer tank pressure.

Regardless of the configuration or type of liquid or gaseous oxidizerused, the assembly of oxidizer tank, pressurizing system and associatedplumbing is typically referred to as the oxidizer section. Collectively,the motor section and the oxidizer section are referred to as the hybridrocket engine, sometimes also referred to as the hybrid rocket motor.

Hybrid rocket engines offer certain advantages over both solid rocketmotors and liquid rocket engines alike. For example, once ignited, asolid rocket motor cannot be stopped until its propellant is exhaustedand it cannot be throttled or restarted. Hybrid rocket engines, likeliquid rocket engines, can be designed for on-command thrusttermination, throttling, and engine restart. Most liquid monopropellantrocket engines use highly toxic, environmentally damaging propellantsthat are now considered too dangerous and to environmentally unsafe forcontinued use.

Compared to most liquid bi-propellant rocket engines, hybrid rocketengines are significantly less mechanically complex, and therefore morereliable and less expensive to develop, manufacture, and operate. Hybridrockets are ideally suited to use propellants that areself-pressurizing, non-toxic, environmentally benign, operate at ambienttemperatures, and require no specialized equipment for handling,transporting, and loading. Furthermore, hybrid rocket engines, due totheir propellants being stored in different states of matter, areinherently immune to explosion. Immunity to explosion is of greatimportance to rocket-powered vehicle designers and operators. Theirsuperior safety, mechanical simplicity compared to liquid bi-propellantrocket engines, and environmental friendliness all translate to improvedreliability as well as lower development, manufacturing, and operatingcosts.

Despite all of their aforementioned advantages, conventionally designedhybrid rocket engines using cast-molded solid fuels likehydroxyl-terminated polybutadiene (HTPB), a form of synthetic rubberthat has been the most studied hybrid rocket engine fuel to date, arerarely if ever employed for applications requiring vibration free,consistent high performance. Unfortunately, conventionally designedhybrid rocket engines using cast-molded HTPB as well as othercast-molded solid fuels, including paraffin wax, polyamides, andthermoplastics have not been able to demonstrate the vibration free,consistent, high performance required for most rocket propulsionapplications.

Excessive vibration and inconsistent performance is even more pronouncedwhen higher energetic additives such as aluminum powder have beenblended into solid fuels like HTPB and paraffin wax. All of thesedisadvantages and inefficiencies are attributable to either the solidfuel material selected or the fuel grain production methods used. Tofully understand the efficacy and advantages of the present invention,it is important to understand these disadvantages in relation tocompeting rocket propulsion systems as well as their respective causes.

Comparative poor hybrid rocket engine performance and their oftenunpredictable, even sometimes dangerous nature can be attributed to: 1)low regression rate, i.e., the rate at which the solid fuel is consumedcompared to solid rocket motors, 2) adverse harmonics build-up inducingunacceptable, sometimes dangerous levels of vibration, 3) excessivesolid fuel waste compared to other rocket propulsion systems, 4) lowspecific impulse (Isp) compared to most liquid bi-propellant rocketengines, and 5) inconsistent, unpredictable thrust performance whichrenders them unusable in clustered (multiple engines per launch vehiclestage or spacecraft) configurations.

1). Low Regression Rate. For a given selection of fuels andoxidizer-to-fuel mass ratios, the thrust generated by a rocket or anytype of reaction engine is approximately proportional to the mass flowrate. In a hybrid rocket engine, mass flow rate is proportional to fuelgrain regression rate. In a classically designed hybrid rocket engine,particularly those using slow burning fuels like HTPB, the burning rateis further limited by the heat transfer from the relatively remote flameto the fuel grain port surface. One of the physical phenomena that limitthe burning rate is the blocking effect that is caused by the injectionof vaporizing fuel into the high-velocity oxidizer gas stream. Given thelinear nature of the oxidizer gas stream, oxidizer/fuel vapor mixing andresulting combustion efficiency is a function of the amount of timeavailable for mixing to occur within a classically designed hybridrocket fuel grain port.

Attempts to increase the burning rate by mixing energetic materials likeAlcoa produced Military Grade 44 aluminum powder (Rockledge, Tex.)(average particle size of 44 microns) with traditional hybrid rocketfuels using cast-molding production methods have been only marginallysuccessful in improving rocket engine performance. Aluminum powder ishighly reactive with oxygen and water. To passivate the material tobecome stable in atmospheric conditions for safe handling, processing,storing, transporting, and use in a rocket engine, the aluminum particleis allowed to form an outer layer of aluminum oxide (alumina), anon-combustible material that when burned acts as a heat sink causing aloss of temperature and energy within the center port.

Nano-scale Aluminum powder is thought to be the next big advancement inboth solid and hybrid rocketry. Elemental Aluminum in nano-scale issignificantly higher in reactivity than micron-scale powder due to itsrelatively high specific surface area. Unfortunately, most attempts tosafely and efficaciously employ this material in both solid and hybridrocketry have not been successful. If allowed to form an alumina shell,effectively consuming a portion of the aluminum core, much of theelemental aluminum's energetic value is lost.

In addition to the challenges associated with obtaining a uniform blendof polymer and metal powder throughout the fuel grain using thecast-molding technique, improved burning rates by use of metal additivessuch as aluminum have only served to exacerbate the problems associatedwith using relatively elastic materials such as HTPB and paraffin waxesas a primary hybrid rocket solid fuel. Moreover, attempts to improve onregression rate further using high energetic material such as ALEXpowder (an ultra-fine aluminum powder produced by the plasma-explosionprocess) have been even less successful and have introduced asignificant potential for spontaneous ignition or explosion stemmingfrom the pyrophoric nature of these ultra-fine powders.

Despite the potential for significant increase in burning rate, on theorder of 30% higher than standard Military grade 44 micron particle sizealuminum powder, employing a material that will spontaneously igniteupon exposure to the atmosphere or explode on contact with water orwater vapor is counter-productive to one of the most significantadvantages of a hybrid rocket engine—its comparative higher safety(i.e., benign failure mode and U.S. Government recognized zero TNTequivalency) compared to other forms of chemical rocketry.

More recent efforts have involved the development of methods tostabilize the nano-scale aluminum particles by encapsulating eachparticle in a polymeric material; thereby, protecting the elementalaluminum from the environment. While some of these approaches such asemersion in benzene followed by compounding with styrene to formgranules of aluminum-styrene have merit and warrant furtherinvestigation, the breakthrough developed by St. Louis, Mo. basedNanoMetallix, LLC is of note and interest. This firm has developed aprocess in which the elemental aluminum particle, measuring 15 nm orless is produced in a reactor simultaneously with the formation of acrystalline polymer outer shell. Thus, the NanoMetallix passivatedmaterial is not only safe to handle, transport, store, and use as rocketpropellant, the particle core remains 99.9% pure elemental Aluminum.

This difference in the combustion scheme of a hybrid rocket enginesignificantly degrades the propellant burning rate compared to a solidrocket motor propellant in which the solid state oxidizer and fuel arein intimate contact. Consequently, the regression rate, usingconventionally molded fuel grain materials like HTPB is typicallyone-tenth or less than that of most solid rocket propellants.

Structurally soft, HTPB with a Young's Modulus varying between 0.0026GPa and 0.00756 GPa is a common polymeric binder used in solid rocketry.It has been the fuel of choice for over fifty years in many U.S.Government sponsored hybrid rocket propulsion research projects. Most ofthis work has involved integrating multi-port configurations into thefuel grain's design to increase the total fuel grain port surface areaas a means to improve regression rate. Unfortunately, improvements inregression rate using multi-port designs have been offset by reducedfuel volume loading, adverse harmonics built-up that induces excessiveand sometimes dangerous levels of vibration, unpredictable thrustperformance, and increased fuel waste. However, excessive vibration,unpredictable thrust performance, and increased fuel waste have alsobeen observed in single port large hybrid rocket engine designs usingboth HTPB as well as faster burning, also structurally soft, paraffinwax with a Young's Modulus of 0.061 GPa. While it is generallyunderstood that regression rate in a hybrid rocket engine is a functionof fuel burn rate and port surface area, the increased regression ratesachieved using multi-port grain configurations have been more thanoffset by reduced reliability, consistency, efficiency, and safety.

2) Adverse Harmonics and Excessive Vibration. In any discussion aboutvibration in a hybrid rocket engine, it is important to keep in mindthat the port within a hybrid rocket fuel grain is the engine'scombustion chamber. Combustion chamber wall integrity is an essentialdesign criterion in any reaction engine. Therefore, it is understandablethat if a combustion chamber wall's structural integrity is degraded orcompromised, chamber performance and reliability would likewise bedegraded or compromised. Logically, an engineer would be reluctant touse a compressible, easily fractured material to fabricate a combustionchamber. But, this is exactly the case when soft, compressible, andfracture prone materials like HTPB and paraffin wax are used toconstruct a hybrid rocket fuel grain and its combustion chamber port orports. To make matters more complex, given the fuel grain is also therocket engine's fuel supply, as fuel is consumed, the port wallcontinually ablates and expands in diameter; thereby, increasingavailable surface area causing an oxidizer-fuel mixture shift fromoxidizer rich to fuel rich combination. Materials such as HTPB andparaffin wax are thought to respond to high pressure gases createdwithin the port by compressing the solid fuel against thehigher-strength motor case; thereby, inducing grain fractures anderosive burning—both common occurrences in large scale HTPB and paraffinwax hybrid rocket engines.

Adverse harmonics exhibited in hybrid rocket engines, particularlypronounced in large-scale variants, is thought to be caused by acompressive-relaxation response by these soft fuels reacting to elevatedchamber pressures, creating a type of trampoline effect. Theseoscillations can build to dangerous vibration levels and even acatastrophic over pressurization event. Cast-molded fuel grains madefrom these materials are also prone to structural flaws such as weakspots, air bubbles, hot spots, and fractures that are also known tocause erosive burning and erratic, unpredictable performance. Fuelfragments breaking free and blocking or temporarily blocking therocket's nozzle have also been recorded. These phenomena are consideredeven more problematic in large hybrid rocket engines, especially thoseusing multi-port designs.

3). Excessive Solid Fuel Waste. A certain amount of residual solid fuelis expected in a hybrid rocket engine. However, in a multi-portconfiguration, the amount of non-combusted fuel that is expelled can besignificant and in certain circumstances a safety concern. In multi-portdesigns, as the burn progresses and fuel is ablated and combusted, thestructure between the ports ultimately losses its integrity untilfailure occurs. In these situations, chunks of non-combusted fuel andwebbing material have been known to break free, partially and sometimescompletely blocking the nozzle, which can cause a serious safetyproblem. In multi-port HTPB fueled hybrid rocket engine designs, thetotal amount of residual and unspent fuel can reach 15% or more.

4). Poor Specific Impulse. Expressed in seconds, specific impulse(usually abbreviated Isp) is a measure of the efficiency of rocket andjet engines. By definition, it is the total impulse (or change inmomentum) delivered per unit of propellant consumed and is dimensionallyequivalent to the generated thrust divided by the propellant flow rate.Typically referenced as performance in vacuum for rockets, Isp is aconvenient metric for comparing the efficiency of different rocketengines for launch vehicles and spacecraft.

Generally speaking, there is an inverse relationship between increasedregression rate and Isp in a hybrid rocket. Whereas, regression ratespeaks to the hybrid rocket engine's volumetric efficiency and thrustoutput as a function of fuel grain diameter, Isp relates more to therocket engine's propellant efficiency. Ideally, rocket engine designersattempt to improve both. However, attempts to improve on hybrid rocketIsp has mainly focused on evaluating and testing different propellantcombination. Whereas, a classical hybrid rocket engine uses a liquid orgaseous oxidizer and solid fuel, past experiments have been conducted onengine's that use a solid oxidizer and liquid fuels. While many of theseachieved very high Isp—in the high 300 seconds (vacuum), they proved tobe impractical for reasons mostly associated with the need to maintain ahydrocarbon fuel as a solid at cryogenic temperatures.

Other approaches have involved blending energetic materials such asaluminum powder into the fuel grain composition to increase Isp.However, obtaining a consistent, uniform mixture has always been achallenge using cast-molding techniques, especially when moldingmulti-port grains. Most conventionally designed hybrid rocket enginesusing nitrous oxide and polymeric fuel like HTPB average Isp is between270 seconds to 290 seconds (vacuum), the higher figure attained with theaddition of aluminum powder as an additive. While higher than most solidrocket motors, this level of performance is significantly lower thancompeting liquid bi-propellant systems using liquid oxygen andhydrocarbon fuels like kerosene that average between 310-340 seconds.

5). Inconsistent Thrust Performance. Inconsistent, unpredictable thrustin a classical hybrid rocket engine is a direct consequence of all ofthe above listed shortcomings and problems. Inconsistent andunpredictable performance makes it impossible for a hybrid rocket engineto be seriously considered for most rocket propulsion applications anduses. Further, many of the causes of inconsistent thrust performance canbe tied to the cast-molding production process used to fabricate hybridrocket fuel grains. HTPB and paraffin wax fuel grains are typicallycentrifugally cast-molded, with the latter containing a small percentageof polyethylene to improve tensile strength. During the HTPBpolymerizing process, small air bubbles are formed and hot spots arecreated due to incomplete mixing and uneven curing. HTPB fuel grainsrequire up to 90 days or more to fully cure, and even then, theirmaterial characteristics change over time. Small air bubbles are alsoformed during the cooling cycle when fuel grains are cast from paraffinwax. Bubble formation is a function of the shrinkage occurring withinthe wax. In an attempt to reduce or eliminate unwanted air bubbles aswell as other types of grain flaws and hot spots, centrifugal castingmethods, taking up to 120 hours to complete, are routinely employed.Even with these measures, air bubbles, structural cracks, hot spots, andother flaws seem to be chronic for fuel grains made using thecast-molding process.

Therefore, it would be highly desirable to develop a solid fuelpropellant and fuel grain architecture-topology that exhibits: 1)flawless composition, 2) a regression rate comparable to solid rocketmotors, 3) significantly improved thrust consistency, 4) more thoroughoxidizer-fuel mixing, 5) greatly improved specific impulse, and 6)minimal vibration—all without compromising the many safety, mechanicalsimplicity, and economic advantages inherent in hybrid rocket propulsionsystems.

SUMMARY OF THE INVENTION

The present invention is a high performance, safe to produce, store,transport, and operate hybrid rocket solid fuel grain made from aformulation of thermoplastic solid fuel and nanocomposite aluminumadditive; and more particularly, fabricated using fused deposition typeadditive manufacturing apparatus.

Additive manufacturing (also referred to as 3D printing or archaicallyas freeform-fabrication) is the official industry standard term (ASTMF2792) for all applications of the technology. It is defined as theprocess of joining materials to make objects from 3D model data, usuallylayer upon layer, as opposed to subtractive manufacturing methodologies.

An exemplary solid fuel grain suitable for use in a hybrid rocket engineand made in accordance with the present invention has a generallycylindrical shape and defines a center port that runs linearly throughits length. The solid fuel grain is formed as a fused stack of layerswith each layer comprising a plurality of abutting, fused concentricgenerally circular, disposed beads of material suitable as a hybridrocket fuel, with each such concentric generally circular shaped beadthus formed, depending upon the additive manufacturing process used, asa ring with a defined cross sectional shape. The plurality ofring-shaped disposed beads is configured in a concentric pattern ofincreasing radii arrayed around the center port or center opening andmade from extruding and disposing a formulation ofthermoplastic-nanocomposite aluminum material.

After being loaded into a hybrid rocket engine's solid section,concurrent with ignition actuation to elevate the temperature within thecenter port above the thermoplastic fuel's ignition or glass transitiontemperature and the nanocomposite aluminum's ignition temperature, aliquid or gaseous oxidizer is introduced into the solid fuel grainthrough one or more multiple injectors along a pathway defined by thecenter port causing a thin layer of the center port wall to phase changefrom solid to gas vapor.

Using a thermoplastic fuel formulation such as 95% by mass AcrynotrileButadiene Styrene (ABS) and 5% NmX-01 nanocomposite aluminum, phasechange will occur from solid to gas vapor along the exposed surface areaof the solid fuel grain port wall. The resulting combined fuel vapor andnanocomposite aluminum then mixes with the oxidizer to form afuel/oxidizer mixture suitable for rocket engine combustion. Theresulting combusted reaction mass is expelled at high temperature andpressure through the rocket engine's nozzle (conventional de Laval oraerospike) at supersonic speed to generate thrust.

Each layer, comprised of a plurality of fused concentric circular beadedstructures of different radii, exhibits a geometry that is designed toexpose more surface area along the center port wall for combustion thanwould otherwise be possible if the center port wall were of a smooth,uniform cast-molded design. During hybrid rocket engine operation,starting with the center port wall and working outward, each beadedconcentric ring structure, after undergoing phase change and ablation,is replaced by the next abutting beaded concentric ring structure. Thisprocess is repeated and persists throughout the rocket engine'soperation until either the oxidizer flow is terminated or the solid fuelis exhausted.

Unlike prior art constructions that attempt to increase regression rateusing cast-molded multi-port grain architecture featuring smooth portwalls at the sacrifice of fuel loading, increased fuel waste, andinduced excessive vibration, the additively manufactured solid fuelgrain of the present invention supports smooth, consistent rocket engineoperation at regression rates previously unobtainable in a single portdesign. Further, by replacing cast-molding production methods withadditive manufacturing methods, grain flaws chronic to both cast-moldedfuel grains made from HTPB and paraffin wax are eliminated.

Another exemplary solid fuel grain suitable for use in a hybrid rocketengine and made in accordance with the present invention is formed asdescribed in the above exemplary example, but with each concentricbeaded ring structure possessing a pattern that both increases thesurface area available for combustion and creates, in its plurality offusion stacked layers, a rifling type pattern within the port walldesigned to induce oxidizer swirling flow around the center port axisline rather than laminar or streamline flow; thereby, creating a vortexwithin the center port to enable oxidizer and gaseous fuel to spend moretime within the center port to mix and combust more thoroughly thanwould otherwise be possible.

Again, as in the above examples, the pattern thus engineered into thefuel grain topology will persist throughout the rocket engine'soperation until either oxidizer flow is terminated or the solid fuel isexhausted. Prior art constructions have employed swirling type oxidizerinjectors to induce vortex flow. However, this technique is onlypartially effective as it cannot generate axial flow throughout thelength of the fuel grain and its center port. In another embodiment,rifling patterns have been imprinted onto the molded fuel grain's portwall as a means to induce axial oxidizer-fuel gas flow. Unfortunately,any vortex generated in this manner is only momentary due to the surfacepattern being quickly ablated and not repeated.

In contrast, the solid fuel grain of the present invention supportssmooth, consistent rocket engine operation at regression rates and atIsp levels previously unobtainable in hybrid rocket engines. Higherenergetic combustion, on the order of 50% or higher than hybrid fuelgrains using aluminum oxide capped micron particle size aluminumadditive, enables rocket engine designers the opportunity to designhybrid rocket engines with significantly reduced propellant loading tomeet dimensional restrictions and performance requirements for manyrocket powered vehicle applications that heretofore, developers wouldnot consider a hybrid rocket engine.

To achieve such a construction, the solid fuel grain is preferablymanufactured using any one of several available fused deposition typeadditive manufacturing machines capable of fabricating articles in aformulation of thermoplastic fuel and nanocomposite aluminum additive.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a solid fuel grain section made inaccordance with the present invention;

FIG. 2 is a top view of the solid fuel grain section of FIG. 1;

FIG. 3 is a sectional view of the grain section of FIG. 1, taken alongline 3-3 of FIG. 2;

FIG. 4 is a flow diagram of an exemplary method for manufacturing thesolid fuel grain section of FIG. 1 using a formulation of thermoplasticfuel and nanocomposite aluminum additive;

FIG. 5A is a side view of a solid fuel grain comprised of a plurality ofsolid fuel grain sections;

FIG. 5B is an exploded side view of the solid fuel grain of FIG. 5A;

FIG. 6 is a perspective view of the plurality of solid fuel grainsections of FIG. 5A wrapped with insulating film;

FIG. 7 is a sectional view of an exemplary rocket incorporating thesolid fuel grain of FIGS. 5A, 5B, and 6;

FIG. 8 is an enlarged sectional view of the motor case of the rocket ofFIG. 7, showing a flame configuration; and

FIGS. 9A, 9B, and 9C are top views of the fuel grain section of FIG. 1as successively consumed by a flame.

FIG. 10 depicts the coordinate system and orientation of the fuel grainfor use with FIGS. 11-14.

FIG. 11 depicts a quarter sectional view of the fuel grain section ofFIG. 1 featuring a concentric corrugation topology grain pattern.

FIGS. 12 and 13 depict quarter sectional views of a fuel grain sectionfeaturing a concentric rifled truncated pyramidal topology grainpattern.

FIGS. 14A and 14B depict a top view and a perspective view of the fuelgrain section of FIG. 1 featuring a concentric rifled polygonal topologygrain pattern.

FIG. 15 is a schematic depicting the production steps and equipmentinvolved in the additive manufacture of hybrid rocket fuel grainsaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a solid fuel grain for a hybrid rocket engineand a method for manufacturing same; and more particularly, a solid fuelgrain manufactured using a fused deposition type additive manufacturingapparatus.

FIGS. 1-3 are various views of an exemplary solid fuel grain section 10suitable for use in a hybrid rocket engine and made in accordance withthe present invention. The fuel grain section 10 has a generallycylindrical shape and defines a center port 16. In this exemplaryembodiment, the center port 16 has a substantially circularcross-section, but the center port 16 could have other geometries, suchas a star, clover leaf, or polygon without departing from the spirit orscope of the present invention.

More importantly, the solid fuel grain section 10 is formed as a fusionstacking of layers with each such layer formed as a series of abuttingfused concentric ring-shaped beads of solidified material 12 arrayedaround the center port 16. As is further described below, whenincorporated into a hybrid rocket engine, an oxidizer is introduced intothe solid fuel grain section 10 along a pathway defined by the centerport 16, with combustion occurring along the exposed surface area of thesolid fuel grain section 10 port wall. Accordingly, each concentricring-shaped structure possesses a geometric pattern 14 that serves toincrease the surface area for combustion compared to a smooth concentriccircular structure or smooth walls as consistent with cast-moldedconstructions. As each such concentric ring-shaped bead undergoes phasechange from either solid to gas or solid to entrained liquid droplet,the abutting concentric bead is exposed to the flame sheet. This processcontinues and persists during the hybrid rocket engine's operation untileither oxidizer flow is terminated or the solid fuel is exhausted.Unlike prior art constructions that improve regression rate byincreasing the surface area exposed to the flame sheet using amulti-port architecture at the sacrifice of fuel loading, the solid fuelgrain of the present invention presents increased surface area as ameans to improve regression rate, but without the disadvantagesassociated with multi-port configurations.

Although the fuel grain section 10 may be manufactured in various sizesor dimensions, in this exemplary embodiment, the fuel grain section 10has an outer diameter, d2, of 19.0 inches. Although a wide range ofdiameters and fuel grain lengths (or sectional lengths) are possible,the center port 16 has an initial diameter, d1, of 4.0 inches in thisexemplary embodiment (although a larger diameter is shown in FIG. 3 toenable a better view of the interior of the fuel grain section 10).

Each of the stacked fused layers in this exemplary embodiment would havean approximate thickness ranging from 0.005 inches to 0.015 inchesdepending upon the additive manufacturing machine layer setting orextrusion dye used, as is further described below.

In this exemplary embodiment, each of the stacked layers 12 is formed bythe deposition of viscous polymer which is extruded following a roughlycircular tool path forming a plurality of solidified abuttingring-shaped beads of material. Viewed in cross section as depicted inFIG. 11, each ring shaped bead of solidified material 90 is oval orelliptical in cross sectional shape, which flattens on its bottom underits own weight as the material cools and flattens on the top as theweight of the next extruded layer of abutting ring shaped beads ofmaterial is deposed above it.

As for the concentric ring-shaped beaded structures thus fabricated, asstated above, the objective is to increase the surface area presented tothe flame zone for combustion within the center port 16 in a mannerwhich is persistent throughout the hybrid rocket engine's operation. Inthis exemplary embodiment, and as illustrated in FIGS. 1-3, the surfacepattern presented to the flame zone is characterized by a series ofprojections and depressions extending radially into the center port inthis case forming elongated undulations that extend axially through thecenter port. These undulations are present in each concentric circularring-shaped beaded structure such that as one ring-shaped beadedstructure is ablated, the next-presented ring-shaped structure isrevealed, presenting the same geometric pattern, but with increasedradii.

In FIGS. 1-3 as well as in FIGS. 11-14B, the individual undulations areidentifiable and have a substantially cylindrical shape. However, inpractice, depending upon the scale and layer thickness, such internaltopology can take the form of a dimple pattern 14 as shown in FIGS. 1-3,a corrugation pattern 92 as shown in FIG. 11, a truncated pyramidalpattern 110 as shown in FIG. 12, a truncated pyramidal pattern 120 asshown in FIG. 13, and an irregular pattern 131 as shown in FIGS. 14A and14B, all of which may or may not be perceptible to a viewer's unaidedeye. Alternatively, the geometric pattern 14, 92, 110, 120, 131 of eachring-shaped concentric beaded structure may take other forms in order toachieve the objective of increasing the surface area available forcombustion that persists throughout the hybrid rocket engine'soperation.

There are many manufacturers producing distinct models of fuseddeposition type additive manufacturing machines in use today capable ofprocessing thermoplastic solid fuel and (with the modifications asdescribed below) a compounded formulation of thermoplastic andnanocomposite aluminum additive to fabricate a hybrid rocket engine fuelgrain consistent with the present invention.

For the exemplary examples shown in FIGS. 1-3, the fused stacked layersof the solid fuel grain section 10 may be formed on a fused depositiontype additive manufacturing machine with sufficient build scale andcapability to produce entire fuel grains or sections which can be joinedduring post-processing. The fused deposition method of 3D printingmachine technology, originally developed by Stratasys, Inc., EdenPrairie, Minn., today is considered a generic form and can be foundunder other trademarked processes such as Fused Filament Fabrication orPlastic Jet Printing. Examples of manufacturers of fused deposition typemachines, of sufficient scale, meeting these criteria include:Cincinnati, Inc. of Harrison, Ohio; Stratasys, Inc., of Eden Prairie,Minn.; Cosine Additive, Inc., Houston, Tex.; and Thurmwood Corp., Dale,Ind.

In addition to fused deposition; there are a number of other additivemanufacturing methods that can be employed to produce hybrid rocket fuelgrains using a formulation of polymer and nanocomposite aluminumadditive without departing from the spirit and scope of the presentinvention, including: Stereolithography, Selective Laser Sintering,Powder Bed Printing, and Inkjet Head Printing.

For the examples shown in the various figures described herein, the fuelgrain is fabricated per the flow diagram shown in FIG. 15 in aformulation of 95% by mass Acrylonitrile Butadiene Styrene (ABS), athermoplastic possessing combustion characteristics desirable for hybridrocket engine fuel and 5% NmX-01 nanocomposite aluminum produced byNanoMetallix, LLC, St. Louis, Mo.

With a Young's Modulus of 2.0-2.6 GPa, ABS is 460 times less elasticthan HTPB and 38 times less elastic than paraffin wax, making it anideal material for fabricating a hybrid rocket fuel grain and itscombustion chamber center port.

Ultra-high energetic nano particle size aluminum, especially aluminumpowder produced without an aluminum oxide shell and passivated (byencapsulating or ‘capping’ the particle in a polymer shell) for safehandling and use, will increase fuel grain burning rate by as much as50% using only a 5% concentration compared to a fuel grain fabricated inABS with a 25% concentration of standard military grade 44 micronparticle size aluminum.

Referring now to FIG. 4, in an exemplary method for manufacturing thesolid fuel grain section 10, the fused deposition additive manufacturingprocess is performed in an additive manufacturing machine 10. Themachine 10 comprises two cartridge mechanisms 20 and 22. One cartridge20 stores a spool of ABS thermoplastic, or a compounded formulation ofABS and nanocomposite aluminum additive, that is used for fabricatingthe solid fuel grain. The second cartridge 22 stores a spool ofwater-soluble disposable material that is used to separate the solidfuel grain section 10 from a support base and support any overhangingstructures specified in the design.

However, other types of additive manufacturing technologies that operatedifferently may be employed without departing from the spirit of thepresent invention. For example, the BAAM™, a giant-scale additivemanufacturing system produced by Cincinnati, Inc., Harrison, Ohio doesnot feature a disposable support material. Instead, a solvent sprayer isused to enable easy separation of the fuel grain from its base as wellas any overhanging structures that are formed.

Once the additive manufacturing process has commenced, monofilamentlines are spooled out from each cartridge 20, 22 and are fed intoliquefiers (not shown) housed in a module 24, with the liquefiersheating the monofilament lines to their respective melting temperatures.The resulting liquefied ABS thermoplastic and support material is thenforced through respective injection nozzles 26, 28 housed in the samemodule 24, so as to form small diameter concentric ring-shaped beads ofmaterial that are disposed upon the support base, in this example, asubstantially flat plastic sheet 30.

In this regard, the module 24 housing the liquefiers and respectiveinjection nozzles 26, 28 is robotically-controlled to allow for movementalong two axes (X, Y) in a plane substantially parallel to theunderlying plastic sheet 30. The plastic sheet 30 is mounted to arobotically-controlled elevator platform 32 that moves along an axis (Z)substantially perpendicular to the module 24 housing the liquefiers andrespective injection nozzles 26, 28. Thus, the elevator platform 32 candrop a distance equal to the specified layer thickness after eachsuccessive layer has been formed.

Thus, the ABS thermoplastic or compounded ABS-nanocomposite aluminummaterial is extruded and placed to form each successive layer ofconcentric fused ring-shaped beaded structures according to the chosendesign of the solid fuel grain section 10, with each successive layerbeing extruded and disposed upon the preceding layer. As eachring-shaped beaded structure cools and solidifies, a fusion bonddevelops between the concentric ring-shaped beaded structure, and aseach plurality of such ring-shaped beaded structures forming layers cooland solidify, likewise a fusion bond develops between the layers.

Once the solid fuel grain section 10 is additively manufactured in thismanner, and removed from the fused deposition additive manufacturingmachine, any build support materials 34 can be either physicallyremoved, or depending on the specific system employed, the fuel grainsection can also be submersed into a water solution to dissolve anybuild support material.

The additively manufactured solid fuel grain section 10 has asubstantially uniform fuel grain density and is substantially free ofvoids. Furthermore, hybrid rocket fuel grains produced in this mannerwill normally require only minimal post-processing surface treatment. Itis important to note that many additive manufacturing systems capable ofproducing hybrid rocket fuel grains consistent with the spirit and scopeof the present invention employ different means to additivelymanufacture solid articles. For example, instead of using line filament,the Cincinnati BAAM uses thermoplastic feedstock in pellet form, similarto those used in injection molding. Stereolithography employs a bath ofliquid photo curable polymer and a UV laser to trace the tool path onits surface to cause the material to solidify. Other additivemanufacturing systems such as Selective Laser Sintering use a powder bedapproach in which a fine layer of polymer powder is laid down to which ahot laser traces the tool path to solidify the material.

Referring now to FIGS. 5A-5B, the individual fuel grain sections 10 a,10 b, 10 c, and 10 d can be assembled and joined together from multipleseparately fabricated sections to form a complete solid fuel grain 40.In this exemplary embodiment, each solid fuel grain section 10 has aheight, h1, of 23 inches, such that the overall height, h2, of thecomplete solid fuel grain 40 is 92 inches. Furthermore, in thisexemplary embodiment, to ensure proper alignment, the topmost solid fuelgrain 10 a has at least one connecting member 100 a extending from itslower surface and at least one cavity 102 a defined in its lower surfacefor receiving a mating connecting member 104 b. Similarly, theintermediate solid fuel grain sections 10 b, 10 c, each have at leastone connecting member 100 b and 100 c, extending from their respectivelower surfaces and one connecting member 104 b, 104 c, extending fromtheir respective upper surfaces, and further each have at least onecavity 102 b, 102 c defined in their respective lower surfaces and atleast one cavity 106 b, 106 c defined in their respective uppersurfaces. Finally, the lowermost solid fuel grain section 10 d has atleast one connecting member 104 b extending from its upper surface andat least one cavity 106 d defined in its upper surface for receiving amating connecting member 100 c in the fuel grain section 10 c.

Accordingly, when heated above its glass transition temperature butbelow the nanocomposite aluminum's ignition temperature, viscous ABS canbe spread or sprayed on the upper and lower surfaces to create a strongfusion bond between the sections 10 a, 10 b, 10 c, 10 d during assembly.In this way, solid fuel grain sections 10 a, 10 b, 10 c, 10 d can bereadily stacked, aligned, and mated to one another to form the completesolid fuel grain 40.

Referring now to FIG. 6, after the solid fuel grain sections 10 a, 10 b,10 c, 10 d are assembled, the solid fuel grain sections 10 a, 10 b, 10c, 10 d collectively define a center port 46 through the solid fuelgrain 40. The solid fuel grain 40 is preferably wrapped in a film 50made of phenol or other suitable thermally resistant material. Placedbetween the inner wall of a fuel motor case (not shown in FIG. 6) andthe outer surface of the solid fuel grain, the film 50 acts as aninsulation layer to reflect heat and prevent damage to fuel motor casesmade from either metal or non-metallic materials such as carbon fiberreinforced polymer composite. Once wrapped in the film 50, the solidfuel grain 40 can be placed into a motor case of a rocket.

FIG. 7 is a sectional view of an exemplary hybrid rocket engine 70housed within an aeroshell 72 to form a complete hybrid rocket poweredvehicle 70 incorporating the solid fuel grain 40 as described above withrespect to FIGS. 5A, 5B, and 6. The exemplary hybrid rocket poweredvehicle 70 generally comprises an aeroshell body 72, a nozzle 82 at onedistal end of said aeroshell body 72, and a payload section 74 at anopposite distal end of said aeroshell body 72. Enclosed within theaeroshell body 72 of the hybrid rocket powered vehicle 70 is a hybridrocket engine including an oxidizer tank 76, a valve 78, a motor case60, and an oxidizer injector 80 housed typically within a forward cap(not shown) that also houses the ignition system (not shown). The motorcase 60 houses a pre-combustion chamber (not shown), a post-combustionchamber 64, and the solid fuel grain 40, which as described above iswrapped in insulating film 50.

The solid fuel grain 40 wrapped in insulating film 50 can be “cartridgeloaded” into the motor case 60 of the hybrid rocket engine.Alternatively, the exemplary solid fuel grain 40 wrapped in insulatingfilm 50 could be wound with a fiber-reinforced polymer composite to formthe motor case without departing from the spirit and scope of thepresent invention. In another exemplary embodiment, the solid fuel grain40 can be inserted into a thermal protection cylinder fabricated frominsulating material such as phenolic or cork without departing from thespirit and scope of the present invention. In yet another exemplaryembodiment, the fuel grain 40 can be formed to embody either or both thepre-combustion chamber and the post-combustion chamber 64 withoutdeparting from the spirit and scope of the present invention.

FIG. 8 is an enlarged sectional view of the motor case 60 of the hybridrocket powered vehicle 70 of FIG. 7, showing the flame zone within thefuel grain center port 46. As shown, an oxidizer 94 (either a liquid ora gas) is injected into the motor case 60 along a pathway defined by thecenter port 46 of the solid fuel grain 40 and flows within the centerport 46, forming a boundary layer 65 bordered by the center port 46wall. The boundary layer 65 is usually turbulent throughout a largeportion of the length of the center port 46. Within the boundary layer65 is a turbulent diffusion flame zone 66 that extends throughout theentire length of the center port 46 and depending upon thecharacteristics of the solid fuel selected, either causing a phasechange to a gas or entrained liquid droplets of fuel to form.Evaporation from the oxidizer/fuel gas/entrained liquid dropletinterface produces a continuous flow of fuel gas that mixes withoxidizer gas at the flame zone 66 to maintain combustion along theexposed surface area of the center port 46 wall. At steady state, theregression rate of the melt surface and the gas-gas or gas-entrainedliquid droplet interface is the same, and the thickness of the gaseousor entrained liquid layer is constant.

Because the additively manufactured port wall surface pattern 14, 91exposed to the flame zone 66 possesses increased surface area comparedto cast-molded constructions, the exemplary solid fuel grain 40 causesincreased regression rate and corresponding increased thrust impulsewithout the decreased fuel volumes associated with multi-port designs.Also, unlike the prior art constructions that increase the surface areathrough a multi-port architecture at the sacrifice of fuel loading, thesolid fuel grain 40 of the present invention allows a smooth burningprocess whereby, as each concentric ring-shaped beaded structure formingeach layer of the fusion stacked layer center port 46 wall is ablated, anew concentric ring-shaped beaded structure, the plurality of whichforms the expanded center port 46 wall is presented to the flame zone66, as shown in FIGS. 9A-9C, illustrating ablation of the center portwall at three different stages. This burning process continues untileither oxidizer flow is terminated or the solid fuel grain 40 materialis exhausted.

FIG. 15 is a schematic drawing depicting the production steps andequipment involved in the additive manufacture of hybrid rocket fuelgrains 40 made from a compounded formulation of ABS thermoplastic andhighly-energetic polymer-capped nanocomposite aluminum. Generally,energetic materials are a class of material with high amount of storedchemical energy that can be released. Highly energetic materials includeultrafine aluminum powder, the particle size of which is in nanoscale.As shown in this exemplary example, ABS thermoplastic 91 is compoundedwith polymer capped nanocomposite aluminum particles 92 to a desiredmixture ratio. As known by those skilled in the art, generally ananocomposite is a material comprising two or more constituent solids,the size of which measures 100 nanometers (nm) or less.

Even though the nano-scale aluminum particle cores 92 a are completelyencapsulated in a polymer based oligomer coating 92 b, and thuspassivated, there remains the possibility that this highly energeticpyrophoric material can still be reactive with oxygen or water vapor. Asa safety precaution, the nanocomposite aluminum, the ABS thermoplastic,and the compounded ABS-nanocomposite materials (i.e., the feedstock tothe additive manufacturing apparatus 90) are stored in containersdesigned to store flammable material 94, preferably infilled with anon-reactive noble gas at all times prior to their use as feedstock inan additive manufacturing process.

In one application, the compounded feedstock is stored within a climatecontrolled environment near the additive manufacturing apparatus 90.

According to one embodiment of the invention, during the fabricationprocess a heavier-than-air shielding gas is used to prevent trapping ofatmospheric air within the fuel gain during 3D printing. Air trapped inthe voids between beaded extrusions and between layers (can range from5% to 15% depending upon the additive manufacturing apparatus used) isnot a problem for fuel grains made from thermoplastic or even whennon-pyrophoric micron scale particle size aluminum is added to theformulation. However, atmospheric air (containing approximately 20%oxygen and varying amounts of water vapor, both of which are highlyreactive with uncapped nanoscale aluminum particles) entrapped in a fuelgrain containing nanocomposite aluminum could present a fire hazard dueto the pyrophoric nature of the material should the polymer capsinsulating the elemental aluminum core become compromised duringproduction.

Thus, a pure heavier-than-air gas, such as argon, carbon dioxide, ornitrogen dioxide that is non-reactive to nanoscale elemental aluminumparticle cores is used to cover the print bed and extruder during 3Dprinting as an added safety measure, particularly when the shielding gasis kept at a lower temperature to aide in the solidification process.

Given that the gas trapped in the voids will react when combusted withinthe rocket engine, the shielding gas should ideally contribute tocombustion, or at minimum, be inert. For example, carbon dioxide willcontribute oxygen to the combustion reaction whereas, argon being aninert noble gas will not.

As a further safety measure, each 3D printed fuel grain or fuel grainsection is shrink-wrapped to encase the fuel grain or fuel grainsections in a thin plastic film to prevent atmospheric exposure prior toits use in a hybrid rocket engine.

In one embodiment a gantry-type fused deposition additive manufacturingsystem like the Cincinnati BAAM is used to 3D print hybrid rocket fuelgrains 40 on the print bed 90 a as shown. The ABS/nAl feedstock 94 isbatch-fed into the argon gas filled dryer unit 95 to remove anyremaining water vapor from the feedstock. The now completely driedABS/nAl feedstock 94 is manually poured into the argon filled Gaylordbox 95 a within the dryer 95, which in turn through piping,pneumatically feeds the material into the additive manufacturingapparatus's extruder 90 b which is mounted on a moveable gantry 90 cover the print bed 90 a. The ABS/nAl feedstock 94 is pneumatically urgedinto and through the extruder 90 b which contains a screw-drive unitwhich grinds the material, and using the friction heat created, elevatesthe material's temperature to achieve the desired viscosity.

The viscous ABS/nAl feedstock is then urged into and through a die whichdeposes a bead of semi-solid material upon either the print bed 90 a orthe proceeding layer of the fuel grain being 3D printed, whichever isthe case. As depicted, multiple fuel grains 40 of two differentdimensions are being 3D printed simultaneously on the print bed 90 a.

Care must be taken to ensure that during compounding as well as duringadditive manufacturing that the polymer capping material whichencapsulates the nanocomposite aluminum is not subjected temperatureselevated above its heat deflection or melting temperature, nor theignition temperature of the nano-scale aluminum particle core.

FIG. 10 depicts the coordinate system and orientation of the fuel grainfor use with FIGS. 11-14 and depicts a preferred build orientationwithin the FDM additive manufacturing machine for fuel grains 93 (FIG.11), 112 (FIG. 12), 122 (FIGS. 13), and 133 (FIG. 14).

FIG. 11 is a quarter sectional view of the fuel grain section of FIG. 1featuring a concentric ring-shaped corrugation build pattern or fuelgrain 92, a port wall surface pattern 91, and several layers of fusedconcentric beads in cross section 90.

FIG. 12 is a quarter sectional view of the fuel grain section of FIG. 1featuring a concentric ring-shaped truncated pyramidal build pattern orfuel grain 113, a port wall surface pattern 110, and several layers offused concentric beads in cross section 111.

FIG. 13 is a quarter sectional view of the fuel grain section of FIG. 1featuring a concentric ring-shaped rifled truncated pyramidal buildpattern or fuel grain 123, a port wall surface pattern 120 with thebuild and surface patterns staggered layer by layer to form in itsplurality a persistent rifling pattern.

FIG. 14A depicts a top view and FIG. 14B a perspective view showing theport wall surface pattern 131 of the fuel grain section of FIG. 1. FIGS.14A and 14B feature a concentric ring-shaped rifled polygonal buildpattern or fuel grain 132 with each such polygonal build patternstaggered and twisted (i.e., rifled) layer-by-layer to form in itsplurality a persistent rifling pattern.

The embodiments of FIGS. 12-14B depict exemplary constructions of ahybrid rocket fuel grain engineered and additively manufactured to bothincrease the amount of surface area available for combustion as a meansto improve regression rate, to improve specific impulse, and to reducefuel waste by inducing oxidizer axial flow within the center port 46(see FIG. 8) to allow more time for oxidizer and fuel gases (or oxidizerand entrained liquid droplets) to mix and combust more thoroughly.

The embodiments of FIGS. 13 and 14A/14B present a persistent riflingpattern to the oxidizer flowing through the center port 46 to induceaxial flow.

A person of art in the field will recognize that the fused depositiontype additive manufacturing apparatuses currently commercially producedand distributed are designed to employ material feedstock in two basicforms: line filament or pellet. Those like the Cincinnati BAAM designedto process feedstock in pellet form are also capable of processingfeedstock material in granule form, provided, the granules are of asmall enough size to flow like sand under gravity into the machine'sextruder.

One of ordinary skill in the art will recognize that additionalembodiments are also possible without departing from the teachings ofthe present invention or the scope of the claims which follow. Thisdetailed description, and particularly the specific details of theexemplary embodiments disclosed herein, is given primarily for clarityof understanding, and no unnecessary limitations are to be understoodtherefrom, for modifications will become obvious to those skilled in theart upon reading this disclosure and may be made without departing fromthe spirit or scope of the claimed invention.

What is claimed is:
 1. A method of making a fuel grain for use in ahybrid rocket engine, the method comprising: compounding a firstmaterial suitable as a hybrid rocket fuel and a second energetic andpyrophoric nanoscale metallic material according to a predeterminedmixture ratio to form a third material; the third material serving asfeedstock material for use in an additive manufacturing apparatus; andoperating the additive manufacturing apparatus using the feedstockmaterial to fabricate a fuel grain comprising a plurality of fusedstacked layers of solidified fuel grain material, each layer of theplurality of layers comprising a plurality of bonded and concentricsubstantially circular ring-shaped beads of different radii and defininga center combustion port.
 2. The method of claim 1 further comprisingdrying the feedstock material and then elevating a temperature of thefeedstock material to attain a predetermined viscosity for the feedstockmaterial.
 3. The method of claim 1 the step of operating comprisingurging the feedstock material through an extrusion die of apredetermined diameter to fabricate the plurality of fused stack oflayers of solidified fuel grain material.
 4. The method of claim 1wherein the first material comprises Acrylonitrile Butadiene Styrene(ABS) thermoplastic having a predetermined monomer composition.
 5. Themethod of claim 1 wherein the second material comprises a plurality ofnanoscale elemental aluminum core particles encapsulated by a cap ofoligomer polymer.
 6. The method of claim 1 wherein the second materialcomprises polymer-capped nanocomposite aluminum powder.
 7. The method ofclaim 1 wherein the first material comprises 95% by mass ABSthermoplastic and the second material comprises 5% by masspolymer-capped nanocomposite aluminum powder.
 8. The method of claim 1wherein a heavier-than-air inert or non-nanocomposite aluminum reactivegas covers a print bed and an extruder of the additive manufacturingapparatus during the step of operating.
 9. The method of claim 8 whereinthe heavier-than-air gas comprises argon, carbon dioxide, or nitrogendioxide.
 10. The method of claim 8 wherein a temperature of theheavier-than-air gas is maintained at a value below a temperature of theof the additive manufacturing apparatus during the operating step. 11.The method of claim 1 further comprising storing the second material inan inert atmosphere for transporting, handling, and compounding with thefirst material, and storing the third material in an inert atmosphereprior to using the third material as the feedstock material during thestep of operating.
 12. The method of claim 11 the inert atmospherecomprising a non-reactive noble gas.
 13. The method of claim 2 whereinthe step of elevating the temperature further comprises processing thefeedstock material to increase internal friction and thereby elevate thetemperature of the feedstock material to achieve the predeterminedviscosity for deposition during the step of operating.
 14. The method ofclaim 1 further comprising during the step of operating, maintaining atemperature of the second material below a heat deflection temperature,melting temperature, and ignition temperature of the second material.15. The method of claim 1 wherein the step of operating furthercomprises: forming a first layer of fuel grain material furthercomprising a plurality of bonded and concentric substantially circularring-shaped beads of different radii and defining a center opening;forming a plurality of additional layers of fuel grain material eachlayer comprising a plurality of bonded and concentric substantiallycircular ring-shaped beads of different radii and defining a centeropening; the first layer and the plurality of additional layers havingsubstantially equal outer and inner diameters or non-equal outer andinner diameters; stacking and fusing the first layer and the pluralityof additional layers together to form the fuel grain such that thecenter opening of the first layer and the center opening of each one ofthe additional layers are aligned to form a center combustion portextending through the fuel grain; the first layer and each one of theplurality of additional layers having an outer circumference definingundulations therein, an inner circumference bounding the centercombustion port and defining undulations therein, and each interveningcircumference between the outer and inner circumferences definingundulations therein; and wherein the undulations present a largersurface area available for combustion within the center combustion portand provide an increased regression rate of the fuel grain materialrelative to a fuel grain lacking such undulations.
 16. The method ofclaim 15 wherein the undulations form a progressive twist through thecenter combustion port thereby forming a helical grooved rifling patternof undulations to induce a swirling gaseous flow within the center port.17. The method of claim 1 wherein a shape of the center combustion portcomprises a circular shape, an oval shape, a polygonal shape, aquatrefoil shape, a star shape, or an irregular shape.
 18. The method ofclaim 1 wherein the fuel grain defines an outer diameter of about 19.0inches and the center combustion port has an initial diameter of about 4inches prior to consumption of the fuel grain material during acombustion process.
 19. Forming a fuel grain segment further comprisinga plurality of fuel grains according to the method of claim 1, furthercomprising vertically orienting a plurality of the fuel grain segmentsand disposing viscous ABS material between a lower surface of a firstfuel grain segment and an upper surface of an abutting second fuel grainsegment to fusion bond the first and second segments.
 20. The method ofclaim 1 wherein each one of the plurality of fused stack layers issubstantially uniform in material composition.
 21. The method of claim 1further comprising shrink-wrapping the fuel grain in a plastic film toinsulate the fuel grain from an ambient atmosphere prior to removing thefuel grain from a print bed of the additive manufacturing apparatus. 22.A method of making a fuel grain for use in a hybrid rocket engine, themethod comprising: compounding ABS thermoplastic with polymer-cappednanocomposite aluminum powder in a ratio of about 95% ABS thermoplasticand about 5% polymer-capped nanocomposite aluminum to form a compoundedmaterial; supplying the compounded material as feedstock material to anadditive manufacturing apparatus; drying the feedstock material;elevating a temperature of the feedstock material to attain apredetermined viscosity; forming a first layer of fuel grain materialfurther comprising a plurality of bonded and concentric substantiallycircular ring-shaped beads of different radii and defining a centeropening therein; forming a plurality of additional layers of fuel grainmaterial atop each preceding layer of fuel grain material, each one ofthe plurality of additional layers comprising a plurality of bonded andconcentric substantially circular ring-shaped beads of different radiiand defining a center opening therein; the first layer and the pluralityof additional layers having a substantially equal outer and innerdiameter or non-equal outer and inner diameter; stacking and fusingtogether the first layer and the plurality of additional layers to formthe fuel grain such that the center opening of the first layer and thecenter opening of each one of the plurality of additional layers arealigned to form a center combustion port extending through the fuelgrain; the first layer and each one of the plurality of additionallayers having an outer circumference defining undulations therein, aninner circumference bounding the center combustion port and definingundulations therein, and each intervening circumference between theouter and inner circumferences defining undulations therein; and whereinthe undulations present a larger surface area available for combustionwithin the center combustion port and provide an increased regressionrate of the fuel grain relative to a fuel grain lacking suchundulations.